A. The answer is - maybe. Not very helpful? There are two ways to go.
One. Check out lots of similar books in your local bookstore or library, and identify four or five publishers who publish work similar to yours. Then go to their websites (99% of publishers have them - find them by Googling their name) and see what their guidelines say. Some will say they consider unagented work - meaning you can send it to them yourself. Others will say they only consider work that is submitted through an agent.
If you are pitching your own book to publishers, be sure to research how to do it well. Elizabeth Lyon's book The Sell Your Novel Toolkit is excellent; a number of other handbooks also teach you how to write a good query letter/outline/synopsis. etc. - all very important when you approach a publisher on your own behalf. Most publishers do offer very clear instructions about what they want to see when you pitch your book. So check their website, read the guidelines, AND GIVE THEM WHAT THEY ASK.
Two. You might choose to look for an agent so s/he can be selling your book while you're working on the next one. But if you want to do it this way, be aware that finding an agent to represent you takes as much time and skill as pitching your book to a publisher – and requires many of the same steps. Check out some of the resources listed here http://www.writers.net/agents.html or pick up a copy of Jeff Herman's Guide to Book Publishers, Editors & Literary Agents 2007: Who They Are! What They Want! How to Win Them Over!
You're more likely to need an agent if you'r pitching fiction that non ficiton, which often sells on the basis of a good query/book proposal.
I know one agent who offers workshops about WHY you don't need an agent - so there's no good answer as to who needs one to sell what type of book.
It may depend on what you are writing, your background/publishing record and if you have good negotiating skills and insights into what makes a good contract.
A quick aside: those who write poetry, short stories, or articles rarely use agents. And there are fewer agents for children’s writers than there are for those writing for adults. The advice coming from agents who do represent children's work is to get one or two books published first, then find an agent.
Learning about what goes on at an agency from the inside out can help you refine your approach, and determine whether this is the route you want to go. A few agent blogs I find useful include:
- BookEnds, LLC - a literary agency (Note - they represent authors who write adult ficiton and nonfiction)
- Nathan Bransford - a Literary Agent ( adult and YA books)
- The Rejector - blog of a literary agent's assistant. Useful insights from someone who is the 'first line of defense' for her boss. i.e. she reads your query before it event gets to the agent's desk.